1. Field of the Invention
The present invention relates to a control system for a plurality of telephone sets and, more particularly, to a control system for a plurality of telephone sets connected as branched to a central office line which allows extension calling, privacy function setting and hold releasing operations between the telephone sets.
2. Description of the Related Art
Home and business extension telephone systems allowing the extension calling, privacy setting and hold releasing operations between a plurality of telephone sets under the control of, for example, a main unit, have been so far well known. This extension telephone system based on the control of the main unit is arranged so that a plurality of extension telephone sets are connected to the main unit. More specifically, in this arrangement, the extension telephone sets are mutually exchangeably connected under the control of the main unit and the state of the respective extension telephone sets is controlled by a control line signal sent from the main unit.
Further, there has been proposed such an extension telephone system wherein a home telephone controller and/or a private or leased control line and an option controller, in pace of the main unit, are used to realize the speech, privacy speech, calling, automatic hold releasing and transfer functions for each of the telephone sets.
However, this telephone system has had a problem that the system requires the controller similar to the main unit and further the control line for control of the respective telephone sets, which results in increased cost.
In view of such a problem in the prior art, it is an obJect of the present invention to provide a control system for a plurality of telephone sets connected simply as branched, which system allows the realization of the same system functions as those of a prior art extension telephone system.